In condensers, boilers, and heat exchangers where two fluids are brought into thermal communication, it is customary to pass one fluid through a plurality of tubes and to pass the other fluid around the exterior of the tubes. The fluid passing around the exterior of the tubes is contained in the shell of the heat exchanger. In most constructions, the tubes are spaced apart and separated from the fluid in the shell by two end plates commonly known as tube sheets. Each tube sheet contains a plurality of holes into which the tubes are inserted. The tubes extend between the tube sheets so that a fluid-tight barrier is formed between the two fluids. To anchor each tube in its respective hole in the tube sheet, the end of the tube is expanded and flared.
It is customary to replace heat exchanger tubes after they have become corroded and pitted and when they commence leaking fluid across the barrier. In replacing tubes, it is common to hydraulically extract the old tubes from the tube sheet and replace them with new tubes.
Hydraulically-operated tube pullers are well known and generally include a tubular housing placed in abutment with a tube sheet so that a gripping member projecting therefrom will be received within the tube to be extracted. The gripping member is radially expanded into biting engagement with the internal surface of the tube by a wedging element. A first piston, which is connected to the wedging element, moves the wedging element relative to the gripping member to effect its expansion. The first piston cooperates with a second piston, which is connected to the gripping member, to conjointly retract the wedging element and the gripping member, resulting in the extraction of the tube from the tube sheet.
Tube pullers of the above-described type are disclosed in, for instance, U.S. Pat. Nos. 2,697,872; 3,369,287; and 3,791,011. The tube pullers disclosed in these patents all suffer from a common problem which seriously impairs their ability to effectively and efficiently perform tube-pulling operations. This problem involves the inability of these tube pullers to prevent the retraction of their gripping members when they have been overexpanded. Such overexpansion of the gripping members typically results from the manual misadjustment of the gripping members during their preexpansion setting, whereby the wedging elements overexpand the gripping members prior to their retraction. If tube-pulling operations are performed while the gripping members are overexpanded, serious damage may be done to the tube sheet, thereby requiring its replacement in addition to the replacement of the tubes.